For applications that involve lifting of objects, the gravitational force is always apparent and requires either passive or active compensation. Active compensation needs external energy that flows into the system, where passive compensation uses the potential energy stored in the system.
In case of active gravity compensation, high force density actuators with high-energy consumption and potentially large moving mass are needed. Applications require often multiple degrees of freedom, which can be reached within one actuator however, usually, for every degree of freedom a separate single degree of freedom actuator is used. This results in an actuation system with an even higher volume/mass and power consumption.
In case of passive gravity compensation, mechanical springs can be used to compensate for the force of gravity. These springs can be designed to compensate for the gravity of a certain weight or a motion profile and therefore, no additional energy is required to overcome this gravitational force. The operation of a mechanical spring is affected by friction, which causes e.g. stick-slip and other non-linear phenomena. The existing mechanical springs can only provide gravity compensation at a predefined horizontal position (defined in the plane perpendicular to the gravitational force). Current applications solve this problem by adding a hinge, which lets the gravity compensation system turn to provide gravity compensation at different horizontal positions.
Mobile arm support systems provide aid during activities of daily living (ADL) such as eating, drinking and, using a computer for people with limited muscle activity. The limited muscle activity can be caused by afflictions such as neuromuscular disease or a stroke, and results in difficulties to overcome gravity or require movement assistance. Therefore, mobile arm support systems use gravity compensation to enhance the human capabilities to perform the ADL more independently. These support systems are used at home and can be mounted on a table, chair or electric wheelchair. In each of these cases no or limited electrical energy is available, therefore, passive (consumption of energy is zero) gravity compensation is beneficial. The currently available passive gravity compensators use mechanical springs, which are pre-stressed. These springs provide adjustable gravity compensation around a single axis. The compensation is adjusted using an electrical actuator, which varies the spring tension.
Using electrical actuators to provide support during ADL results in bulky and cumbersome arm supports, which is disadvantageous to use at home. The arm support becomes bulky because several single degree of freedom actuators are used for the shoulder joint alone. Utilizing multiple degrees of freedom actuators can decrease the heaviness and voluminous of the arm support, where spherical actuators could be used because they can mimic the shoulder joint. However, spherical actuators cannot provide the torque density needed to support the shoulder joint.
What is needed is a device that provides positioning and gravity compensation, and enables multiple axis ranges of motion without using hinges or mechanical springs.